Lobe-type rotary pumps are useful in pumping both thin and highly viscous products. Operation of lobe-type rotary pumps involves entry of the product into a suction port where it is gently drawn into pockets formed between rounded rotor lobes and the rotor case. The lobes push the product 180.degree. C. around the interior of the curved contour of the case and out the discharge port. Each rotor equipped normally with two or more lobes moves in an opposite direction around parallel displaced axes to provide a smooth, uniform flow of product. Rounded lobes have no sharp cutting edges so that delicate handling of products with particles in suspension can be done while minimizing shear damage. The tolerances and timing actions are such that wear to pump parts is critical to efficient operation. Further, pumps of this type are subject to gradual or sudden surges which may block or back-up the flow being pumped by the rotating lobes.
In the prior art of rotary pumps generally such as a single shaft drive gerotor or vane design, provisions have been made to provide an overpressure relief function. U.S. Pat. No. 3,655,299 is pertinent to the very broad concept of an internal bypass function of a pump wherein a front plate includes a Belleville spring to bias the plate against an edge of an inner one of concentric pump rotors. When the pump fluid pressure exceeds a maxmium amount, the pressure force acting on the plate exceeds the bias force of the spring, moving the plate away from the rotor edge and allows fluid to flow from one chamber to the other chamber effectively stopping pumping action. As pressure subsides, the plate again is urged against the one rotor edge closing the gap clearance and normal pumping action resumes. A wearplate function is not stated.
Other pressure bypass constructions are seen in U.S. Pat. Nos. 3,806,283 (diaphragm moving away from chamber slots); in 4,336,004 and 4,398,871 (sensor and control valve with end plate movement); and 4,408,963 (bypass flow back to inlet to give constant rate of fluid flow to the system).